A block copolymer consists of two or more polymeric chains (blocks), which are chemically different and covalently attached to each other. Block copolymers are being suggested for many applications based primarily on their ability to form nanometer scale patterns. These self-assembled patterns are being considered as nanolithographic masks as well as templates for the further synthesis of inorganic or organic structures. Such applications are made possible by taking advantage of contrasts in chemical or physical properties that lead to differential etch rates or attractions to new materials. New applications in, for example, fuel cells, batteries, data storage and optoelectronic devices generally rely on the inherent properties of the blocks. All of these uses depend on the regular self-assembly of block copolymers over macroscopic distances.
Block copolymer thin films are of particular interest because of the possibility of obtaining multi-dimensional patterns with very high registry and regularity. In particular, they provide access to a length scale that is not available to traditional lithographic techniques. One focus has been on the patterning of microelectronics components on a length scale inaccessible by optical lithography. Many device structures are simply miniaturized versions of current electronics. For instance, high density hard drives can be made by using the polymer nanodomains to pattern magnetic bits with a significantly greater number of bits per unit area than the current optical lithographically patterned drives.
Traditional microelectronics fabrication has relied on approaches involving the direct patterning of subsequent layers. Trends to smaller feature sizes, as exemplified by Moore's law, with inherently decreased diffusion distances and increased surface/volume ratios are rapidly approaching the limits of traditional optical lithography. While several alternatives have been introduced, many are prohibitively expensive. Self-assembled block copolymers as lithographic masks offer a potentially inexpensive alternative. In this case, the block copolymer thin film is used to transfer a dense set of nanoscale patterns into the desired material. The goal is generally to achieve highly uniform structures with high aspect ratios for application in a variety of devices from biochips to quantum dot arrays to transistors.
In the thin film state, the block copolymer nanodomain formation takes place relative to the surfaces of the film. The nanodomains tend to form with a particular orientation to the substrate surface. In the case of shapes with a long axis (e.g. cylinders and lamellae), the orientation of the long axis with the surface is a major characteristic of the film. Cylinders lying parallel to the surface and lamellae standing perpendicular may each be of interest in the patterning of nanowires. Upright cylinders, lamellae and spheres may be of interest in the patterning of arrays for use, for example, in data storage.